Add task

src/SUMMARY.md

@@ -28,10 +28,11 @@
         - [Join](advanced/composing/join.md)
         - [Select](advanced/composing/select.md)
     - [Future Tree](advanced/composing/tree.md)
+    - [Procedural](advanced/composing/procedural.md)
 - [async and await](advanced/async_await/readme.md)
     - [Size](advanced/async_await/size.md)
     - [Exercises](advanced/async_await/exercises.md)
-- [Task]()
+- [Task](advanced/task/readme.md)
 - [Mutex]()
 - [Stream]()
 

src/advanced/composing/procedural.md

@@ -0,0 +1,9 @@
+# Procedural
+
+A Future (tree) is procedural.
+
+Externally, as a whole, a Future is "single-threaded" -- at any given time, a
+Future should be polled in *one* thread.
+
+Internally, as a tree of Futures, the Futures involved in the "current external
+poll" should be polled *one by one*, in the given thread.

src/advanced/task/readme.md

@@ -0,0 +1,19 @@
+# Task
+
+## Internally
+
+Runtimes use Tasks internally to manage Futures. A Task is an object which
+contains a Future and "everything else" needed to manage the Future.
+
+
+## Externally
+
+While "Task" is internal to a runtime, knowing that it exists can help us
+understand the runtimes better, and thus use them well.
+
+Like a Future, a Task is also [procedural](../composing/procedural.md). To have
+Futures polled in a "multi-threaded" way, they must be spawned (see
+[smol::spawn][1] or [tokio::task::spawn][2]).
+
+[1]: https://docs.rs/smol/latest/smol/fn.spawn.html
+[2]: https://docs.rs/tokio/latest/tokio/task/fn.spawn.html